The present application relates to a communications system of noncontact type in which a communications terminal (or a transponder) containing no radio generating source transmits data with a device (or a reader/writer) that is a communications mate in a wireless manner and a memory card having a radio section that operates as a transponder in a noncontact communications system. For example, the present application relates to a communications system and a memory card that are configured to execute data communication based on a reflected-wave transmission scheme by use of die transmission of non-modulated carrier from a reflected-wave reader side and the absorption and reflection of received radio based on an antenna terminal operation on the side of a reflector.
More specifically, the present application relates to a communications system and a memory card that are configured to establish connection through a service entry sequence in which a reader/writer transmits a beacon frame at a predetermined period to tell an own station service area and a transponder returns a response frame for the beacon frame and relates and, more particularly, to a communications system and a memory card that are configured to reduce power dissipation on the side of a transponder involved in a connection establishment procedure of a service entry sequence for example.
A noncontact communications system called RFID (Radio Frequency Identification) is known as a communications system that is configured to transmit data by radio without having an own radio generating source. The RFID is also referred to as an ID system and a data carrier system. Commonly used worldwide is an RFID system or simply RFID, a recognition system based on high frequency (radio). The RFID system is made up of a transponder called a tag and a reader/writer for accessing the transponder. The transponder passively operates on the radio supplied from the reader/writer as an energy source and the reader/writer reads information from the transponder and writes information thereto.
The noncontact communications methods based on the RFID include electrostatic coupling, electromagnetic inducting, and radio communicating, for example. The RFID system based on electromagnetic inducting is made up of a primal coil in the reader/writer side and a secondary coil on the transponder side. The reader/writer transmits an AC signal of a predetermined carrier frequency to supply power and the transponder side receives this AC signal at the secondary coil and rectifies the received AC signal to take out power for operation. The transponder can transmit data to the reader/writer by executing modulation, such as amplitude modulation, by load switching of the secondary coil.
With the RFID system based on radio communicating, the transponder has a reflector configured to transmit data by a reflected wave obtained by modulating a non-modulated carrier and the reader/writer has a reflected-wave reader configured to read data from a modulation reflected-wave signal received from the reflector, thereby executing reflected-wave transmission also called backscatter. Receiving a non-modulated carrier from the reflected-wave reader, the reflector modulates the reflected wave on the basis of antenna load impedance switching for example, thereby superimposing data onto the carrier. Namely, because the reflector requires no carrier generating source, a data transmission operation can be driven at low power dissipation. The reflected-wave reader receives the modulated reflected wave thus obtained and demodulates and decodes the received wave to get transmission data.
Basically, the reflector has an antenna for reflecting radio that is an incident continuous wave, transmission data generating circuit, and an impedance variation circuit for varying a load impedance of the antenna in correspondence with transmission data (refer to Japanese Patent Laid-Open No. Hei 1-182782, referred to as Patent Document 1 hereinafter for example). The impedance variation circuit is an antenna switch for switching between open and ground of the terminal of the antenna, for example. This antenna switch may be made up of a CMOS (Complementary Metal Oxide Semiconductor) transistor by building the antenna switch into a circuit module. It is also practicable to configure the antenna switch by a GaAs (Gallium Arsenide) IC (Integrated Circuit), separate from the circuit module, thereby realizing high-speed switching at low power dissipation. In the latter, a data transmission rate based on reflected-wave modulation is enhanced, while the power dissipation is suppressed below several 10 μW. Therefore, considering a power dissipation of about several hundred mW to several W in a wireless LAN (Local Area Network), the reflected-wave communication can be said to be remarkably higher in performance than the average power dissipation of a general LAN (refer to Japanese Patent Laid-open No. 2005-64822, referred to as Patent Document 2 hereinafter, for example).
Because the transponder carrying a reflector executes an operation of reflecting received radio, the transponder is not regarded as a radio station, so that this transponder can be advantageously handled as outside the regulations imposed on radio communication. While noncontact communications systems in the past use frequencies of several MHz to several hundred MHz (13.56 MHz for example), the reflected-wave transmitting can use a high-frequency band of 2.4 GHz (microwave) called ISM (Industry Science and Medical Band), for example, thereby realizing high-speed data transmission.
In one tropical exemplary use of a noncontact communications system, a host device incorporating a transponder, such as a reflector, is arranged on the reading surface of a reader/writer, such as a reflected-wave reader as shown in FIG. 11, thereby executing information reading and writing on the transponder.
The transponder can get an electrical power by rectifying a carrier transmitted from the reader/writer; but this power is not enough for the transmission of large amounts of data at high frequencies. Namely, with the transponder, an electrical power for generating a carrier at the time of transmission is not required, while an electrical power necessary for a transmitting operation for modulating a reflected wave and a receiving operation for demodulating and decoding a modulated signal from the reader/writer is supplied from the host device.
For example, if the transponder is built in portable information terminals, such as a digital camera and a mobile phone with camera and a mobile terminal device, such as a portable music player, which are minimized in power dissipation and the reader/Titer is built in information equipment based on stationary household appliances, such as a television set, a display monitor, a printer, a personal computer (PC), and a VTR (video Tape Recorder), a DVD (Digital Versatile Disc) player, then bringing the transponder and the reader/write to proximity of each other allows uploading of image data taken by a mobile phone with camera or a digital camera to the PC in a noncontact manner, thereby storing, displaying, or printing the uploaded image data.
A memory card having capabilities of transponder, such as a reflector, is proposed (refer to Japanese Patent Laid-open No. 2006-216011, referred to as Patent Document 3 hereinafter, for example). Such a host device loaded with such a memory card as a digital camera or a mobile phone can access the loaded memory, card via a wired interface. On the other hand, an external device, such as a PC or a printer, can read data from a loaded memory card via a reflected-wave transmission route independently of the host device loaded with the memory card. Namely, a noncontact communications operation can be controlled out of control of the host device loaded with a memory card, so that the host device does not have to install any driver software necessary for controlling noncontact communication.
Data transmission between transponder and reader/writer requires to establish connection therebetween. For one of communications establishing procedures, a service entry sequence is proposed in which the reader/writer transmits a beacon frame at certain intervals to tell a service area of own station.
FIG. 12 shows a communications control sequence for executing reflected-wave transmission between transponder and reader/writer by use of the above-mentioned service entry, sequence.
The reader/writer periodically transmits a beacon frame to tell the service area of own station. Also, in an entry period to be provided after the transmission of a beacon frame, the reader/writer continues to transmit a non-modulated carrier for operating the transponder.
On the other hand, the transponder receives a beacon frame to know the existence of the reader/writer and uses the non-modulated carrier received within the entry period to return an enter frame in response to the received beacon frame.
FIG. 13 shows a sequence in which a communications operation is started between reader/writer and transponder by use of the service entry sequence shown in FIG. 12.
The reader/writer executes the transmission of beacon frames at certain time intervals in an intermittent manner. The transponder may not receive beacon frames outside a radio reach range; but, when the transponder gets in the radio reach range and a beacon frame comes, the transponder executes the processing of receiving the beacon frame.
On the basis of the information written to the payload of the received beacon frame, the transponder gets information about communications frequency channels to be used and information such as the ID unique to the reader/writer. In order to establish connection with the reader/writer, the transponder returns an entry frame in an entry period by use of a non-modulated carrier.
On the basis of the written contents of the entry frame, the reader/writer gets the information such as the ID unique to the transponder and communications parameters that can be set. In order to make communication with the transponder, the reader/writer transmits a connection request frame with specified information such as communications parameters written to the payload. In response to this connection request, the transponder returns a connection answer frame with the information such as a connection result written to the payload, upon which the connection between transponder and reader/writer is established. While the connection is established, information is read from the transponder and written thereto by repeating the transmission of command frames from the reader/writer and the return of response frames by the transponder.
The reader/writer does not always have to continue to transmit beacon frames at certain time intervals. For example, the reader/writer may establish connection through the same connection request/answer sequence as described above after the transmission of a beacon frame (or an irregular transmission request signal) when to read information from the transponder. Namely, the reader/writer can suppress the transmission of unnecessary beacon frames and the transmission of the non-modulated carrier during an entry period.
On the contrary, on the transponder side, the timing of the transmission of a beacon frame from the reader/writer is unclear, so that the transponder has to execute a wait operation. For example, with the system configuration (described above) in which the transponder such as a reflector is incorporated in a memory card, which is loaded on a battery-driven host device such as a digital camera or a mobile phone, it is unclear for the memory, card to know when the host device is brought to the proximity of the reader/writer to start data transmission. Therefore, it is necessary for the transponder built in the memory card to always wait for the arrival of a beacon frame.
Although the transponder need not the power for generating a carrier at the time of transmission, the transponder uses the electrical power of the host device for a transmitting operation to modulate a reflected wave and a receiving operation to receive an information signal from the reader/writer (as described above). Hence, the power dissipation in the transponder's wait state presents a serious problem for the host device.
Especially, for applications in which high-speed data transmission of about 20 Mbps in 2.4 GHz band is executed, it is unrealistic to put the memory card always in a wait state.